A conventional air conditioning apparatus is described in Japanese Laid-open Patent Publication No. 2007-163099. In this air conditioning apparatus (heat pump system), an indoor unit (usage unit) having an indoor expansion valve (usage-side expansion valve) and a usage-side heat exchanger is connected to an outdoor unit (heat source unit) which has a compressor, a heat-source-side heat exchanger, an outdoor expansion valve (heat-source-side expansion valve), a bypass refrigerant circuit (intake return tube), and an accumulator. In this air conditioning apparatus, during air-warming operation (heating operation) in which the usage-side heat exchanger functions as a refrigerant radiator, the usage-side expansion valve connected to the outlet of the usage-side heat exchanger is controlled so that the degree of subcooling of the outlet refrigerant, which is the degree of subcooling of the refrigerant in the outlet of the usage-side heat exchanger, is equal to a predetermined target degree of subcooling of the outlet refrigerant. The accumulator in this arrangement is connected to the inlet of the compressor. The heat-source-side expansion valve is connected to the inlet of the heat-source-side heat exchanger for heating operation. The intake return tube connects the inlet of the accumulator with the portion of a liquid refrigerant tube that extends from the usage-side expansion valve to the heat-source-side expansion valve, the liquid refrigerant tube connecting the usage-side heat exchanger and the heat-source-side heat exchanger.